This invention relates generally to armoured electric cable and more particularly to a cable having increased tensile strength.
Tensile strength is a requirement for all electric cables, but is a particular concern for large power cables having long vertical runs such as in mineshafts and high rise buildings. In order to provide a sufficient safety factor, it is necessary that the tensile strength of the cable be several times the total weight of the run of cable. In the past, if the conductors themselves have not provided sufficient tensile strength it has been known for horizontal cable runs to clamp an auxiliary tensile member such as a steel wire cable to the power cable to provide additional strength. However, this arrangement shown in General Electric brochure WCD-154 dated Mar. 31, 1983, page 5 has the disadvantage of being cumbersome and costly to install. More recently, as shown in The Okonite Company Bulletin SFC'84, page 32, a cable has been provided in which the tensile member is embedded in an extruded insulating jacket in a figure-8 configuration. It is also known to embed ground wires and shielding wires in extruded jackets as shown in U.S. Pat. No. 4,360,704 to Madry which issued Nov. 23, 1982 and Cables Corporation Product Catalog, pages 9 and 11.
For vertical installations, power cables are known having a steel protective armour cover such as steel wire armour cable shown in Canada Wire CN Tower brochure dated 1973 and VERLOK (Trade Mark of the applicant) cable shown in the applicant's Canadian patent No. 990,374 which issued June 1, 1976. Of course, the steel armour coating adds to the weight of the cable and thus more tensile strength is required. As discussed in Canadian patent No. 990,374, longitudinal slippage between the extruded jacket and the armour cover was a concern. However, slippage between the tensile members and the armour cover or the central conductors is also very undesirable and dangerous.